The present invention relates to recording/reproducing devices for optical disks with wobbled grooves where address information is recorded, and particularly relates to demodulation circuits for demodulating addresses.
On an optical disk such as a DVD (digital versatile disk)+R disk and a DVD+RW disk, an address is recorded by utilizing a sinusoidally serpentine groove (wobbled groove). The address thus recorded is called address in pre-groove (ADIP).
A signal obtained by irradiating the optical disk with light and converting the reflected light into an electric signal is a wobble signal modulated by a wobbled groove. Use of this wobble signal enables highly-accurate address detection.
FIG. 18 is a graph showing an example of a waveform of a wobble signal. As shown in FIG. 18, the wobble signal is modulated by a binary phase shift keying (BPSK) scheme. Demodulation circuits for demodulating such wobble signals are disclosed in Japanese Examined Patent Publication (Kokoku) No. 6-19898, Japanese Unexamined Patent Publication (Kokai) No. 5-260413 and Japanese Unexamined Patent Publication (Kokai) No. 2001-126413.
FIG. 19 is a block diagram showing a configuration of a known demodulation circuit. A demodulation circuit 900 shown in FIG. 19 includes a band-pass filter (BPF) 3, a multiplier 4, a low-pass filter (LPF) 5, a binarization circuit 6 and a phase adjustment circuit 910.
An LPF 1 removes noise at high frequencies from a wobble signal WB input to the LPF 1 and outputs the resultant signal. An A/D converter 2 converts this signal into a digital signal. The BPF 3 further removes noise from the output from the A/D converter 2 and outputs the resultant wobble signal WDF to the multiplier 4 and the phase adjustment circuit 910. The phase adjustment circuit 910 receives a carrier wave CB generated by a PLL. The carrier wave CB has the same period as the wobble signal WB. The phase adjustment circuit 910 detects a phase difference between the wobble signal WDF and the carrier wave CB, adjusts the phase of the carrier wave CB so that the phase of the carrier wave CB coincides with that of the wobble signal WDF, and outputs the result to the multiplier 4.
The multiplier 4 multiples the output of the BPF 3 and the output of the phase adjustment circuit 910 together and outputs the result to the LPF 5. The LPF 5 smoothes the output of the multiplier 4. The binalization circuit 6 compares the output of the LPF 5 with a threshold value and outputs the comparison result. In this manner, an address recorded on a wobbled groove is demodulated.
FIG. 20A is a graph showing a relationship between the carrier wave CB and the wobble signal WDF in a case where the phase of the carrier wave CB slightly leads the phase of the wobble signal WDF. FIG. 20B is a graph showing a relationship between the carrier wave CB and the wobble signal WDF in a case where the phase of the carrier wave CB slightly lags behind the phase of the wobble signal WDF.
The phase adjustment circuit 910 detects the time from when the wobble signal WDF exceeds a predetermined value to the rising of the carrier wave CB. In the case of FIG. 20A, the phase adjustment circuit 910 detects a value P1 close to 360° as a phase difference. In the case of FIG. 20B, the carrier wave CB is slightly behind the case of FIG. 20A, but the phase adjustment circuit 910 detects a small value P2 close to 0° as a phase difference.
Accordingly, if the phase difference between the wobble signal WDF and the carrier wave CB input to the phase adjustment circuit 910 shown in FIG. 19 is small, the detected phase difference changes considerably only by a small variation in signal phase due to the influences of offset variation, amplitude variation, jitter and others in some cases. In such cases, the phase adjustment circuit 910 cannot cope with this change, and therefore does not appropriately adjust the phase of the carrier wave CB, resulting in failure in address demodulation.